This invention relates to systems and methods for processing semiconductor wafers.
In semiconductor fabrication, various layers of insulating, conducting and semi-conducting materials are deposited to produce a multilayer semiconductor device. Using various fabrication techniques such as coating, oxidation, implantation, deposition, epitaxial growth of silicon, lithography, etching, and planarization, the layers are patterned to form elements such as transistors, capacitors, and resistors. These elements are then interconnected to achieve a desired electrical function in an integrated circuit (IC) device.
In many operations, residual unwanted materials such as post-etch/post-strip chemicals and slurry particles accumulate on the surface of a wafer. If left on the surface of the wafer for subsequent fabrication operations, these unwanted residual materials and particles may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable.
To illustrate, fabrication operations such as plasma etching, stripping and chemical mechanical polishing (CMP) may leave unwanted residuals on the surface of the wafer. These unwanted residuals may be removed using water washing, chemical washing, sonic washing (for example Megasonic and ultrasonic), and brush cleaning with deionized (DI or DIW) water, or a separate post-CMP cleaning. The post-CMP step is typically achieved by mechanical brush cleaning, using a polyvinyl alcohol (PVA) brush or sponge and DI water, or potassium or ammonium hydroxide as the cleaning agent. Other surface preparation processes can include chemical processes using various liquid chemicals.
After the cleaning operation, a rinse is applied with DI water and a drying process is performed. One of the substrate drying processes conventionally known in the art is a spin dry process for rotating a substrate at high speeds to spin off water from the surface of the substrate by centrifugal force in a single-wafer type substrate processing apparatus for processing substrates one by one.
One purpose of drying the substrates is to remove water on the substrates after cleaning. Currently several drying methods have been used in electronic component industry. The methods include a spin-rinse dry method, a hot water slow pull method, a Marangoni-type process, and an isopropyl alcohol (IPA) process.
FIG. 1 shows an exemplary prior art of typical spinning apparatus. A wafer 10 is positioned above a wafer chuck 12, both of which are contained in a shroud 14. The chuck 12 is connected to one end of a spindle shaft 19, while the other end of the spindle shaft 19 is connected to a pulley 20. The shaft 19 is centered in a spindle housing 18 using a plurality of spindle bearings 16. The pulley 20 is driven by a belt 22, which in turn is connected to a motor pulley 24. The motor pulley 24 is connected to a motor 26 which, when activated, rotates the pulley 20 to rotate the shaft 19 and the chuck 12 to spin the wafer 10 resting above the chuck 12.
The spin-rinse dryer uses centrifugal forces to remove water from substrate surfaces. However, spin-rinse dryer is known to have problems such as water spotting, static electric charge build-up, and stress-induced substrate damage due to high speed spinning about 2500 RPM. In the hot water slow pull method, the substrates are immersed in a hot water bath, which is heated to 80–90° C., and then slowly pulled from the bath. When a substrate is pulled from the bath, a thin water film is formed on the surface of the substrate. Then, the thermal energy stored in the substrate evaporates the thin water film. For successful evaporation, the rate at which the substrate is separated from the bath must be matched to the evaporation rate. The hot water process has several shortcomings. When the substrate has a non-homogeneous surface, partly hydrophobic and partly hydrophilic, the substrate is likely to have watermarks or stains thereon. Further, condensation of water vapor on the substrate after the substrate is pulled from the hot water may produce watermarks or stains on the substrate.
Since spin dryers or IPA vapor dryers cannot completely remove watermarks that occur on a wafer surface or between patterns, Marangoni dryers have been developed. The Marangoni dryer uses a difference between surface tenses of the IPA and water. The Marangoni-type process involves the introduction of a polar organic compound which dissolves in the liquid and thereby reduces the surface tension of the liquid. U.S. Pat. No. 6,027,574, entitled “METHOD OF DRYING A SUBSTRATE BY LOWERING A FLUID SURFACE LEVEL”, shows a Marangoni-type process. According to the Maringoni principle, fluid flows from low surface tension region to high surface tension region. In the Marangoni-type process, while the substrate is separated from the bath containing water that is at room temperature, the water is driven away from the substrate because of the Marangoni effect. To avoid condensation of water vapor on the surface of the substrate, the Marangoni-type process does not use hot water. After wafers are rinsed out by de-ionized water, the IPA vapor is fed to an upper interior space of a rinsing bath and the DI water is slowly withdrawn. Thus, the water is eliminated from a wafer surface. When the DI water is completely drained, the nitrogen of high temperature is fed into to evaporate the DI water remaining on the wafer surface. If the evaporated DI water and residues including particles are not fully issued out, they can cause the irregular liquid flow (turbulence) in the rinsing bath together with the nitrogen, so that the wafer surface is not uniformly dried and the water remains at a portion contacting with a wafer guide. In addition, since the Marangoni dryer cannot fundamentally prevent oxygen from reacting on the wafer, it cannot suppress formation of an oxide layer.
As noted in U.S. Pat. No. 6,625,901, several issues arise with conventional Marangoni-type process. First, the drying speed of the process is low, because the substrate is dried at room temperature, and the chamber is purged of the remaining IPA vapor for an extended period of time (3–5 minutes) after being removed from the water. Accordingly, drying cost is high. Second, although room temperature water is used, there is still a condensation problem during and after the separation of the substrate from the water. Water vapor condenses on the substrate and forms micro droplets that leave a residue behind, causing defects in subsequent manufacturing processes. Fourth, purging of IPA while the substrate is dried in the chamber may cause condensation of water vapor.